Historically, the acquisition of seismic data was accomplished by creating an explosion that propagated a broad frequency spectrum of seismic energy into the ground. The energy carried down into the ground reflecting and refracting off and through the various strata below the surface and the returning wavefield was recorded. This type of seismic acquisition was slow and dangerous.
In the 1950's, Conoco developed sweep-type vibrators that reduced the energy intensity of the explosion by spreading the smaller energy over a longer period of time as shown in U.S. Pat. Nos. 2,688,124, 3,024,861, 3,073,659, 3,159,233, 3,209,322, and 3,293,598, etc., for example. This certainly improved safety while still providing a frequency spectrum of energy into the ground. Sweep-type vibrators have now been in common use for over 50 years. The seismic surveys accomplished with sweep-type seismic sources are reliable and consistent and, most importantly, are safer than taking explosives into the field. However, it has long been recognized that high frequency energy provides a level of detail in the seismic record that is highly desirable, but the intensity or amplitude of the high frequency energy in the data record has been less than desirable.
Conventional efforts to increase the recordable high frequency energy have been primarily focused on providing longer sweeps or to lengthen the proportion of the sweep time for which the higher frequency energy is delivered into the ground. As a sweep-type vibrator delivers the seismic energy into the ground, it records each sweep and computes an approximate ground force delivered into the ground for use by a feedback circuit to control the vibe. This ground force approximation is used in subsequent analysis in seismic data processing. Conventional vibrator technology uses a weighted-sum method to approximate the “ground force” during a sweep. In 1984, Sallas derived the weighted-sum method to approximate the true ground force. See J. J. Sallas, Seismic Vibrator Control and the Downgoing P-Wave, GEOPHYSICS 49(6) (1984) 732-40. The weighted-sum method assumes that a baseplate acts as a rigid body, and that a full coupling between the baseplate and the ground is achieved. Under these assumptions, the weighted-sum ground force is obtained by summing the weighted baseplate and reaction mass accelerations. The Sallas approximation or equation may be written as:−Fg=MrAr+MbAb,where Mr=Mass of the reaction mass (kg); Mb=Mass of the baseplate (kg); Ar=Reaction mass acceleration (m/s2); Ab=Baseplate acceleration (m/s2); and Fg=Compressive force exerted on the earth by the baseplate (N). This is normally reported as the ground force of the vibrator.
The dynamics of vibrator systems seems to inherently limit the power that is deliverable into the ground at high frequency. A low frequency is delivered by a longer, slower stroke of the reaction mass while a higher frequency stroke is fast and typically shorter in length. While the Sallas approximation indicates that a fast stroke of shorter length provides equal force to the ground, the absence of the higher frequency data in the data traces or records from the field could mean that either the true force is not what is approximated by the Sallas equation or that consistent force across a broad frequency spectrum does not deliver consistent energy delivery across a broad frequency spectrum.